I am simulating the fluid flow inside a solar reactor. The reactor has two inlets and two outlets. The inlets and outlets are circular pipes that open into vertical rectangular channels (please see the attached picture). I am not modeling the pipes and my domain starts/ends at the intersection of the circular pipe with the vertical channel. The computational domain contains a porous cylinder (As seen by the solid blue color) and non porous fluid domains too. The physical problem that I am solving can be described as follows. The entire reactor is initially filled with static air at room temperature and atmospheric pressure. At t>0, Nitrogen and CO2 flow into the reactor from their respective inlets, establish a fluid field and go through the outlets. So, as time progresses, initial air gets displaced. So, the steady state of the reactor (if exists) should not have air. Here are a few questions that I have -

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1) The Reynolds number based on the hydraulic diameter is ~2000 for one inlet and ~6000 for the other. So, ideally I should use a turbulence model for accuracy/convergence. But if you see the flow path after the inlets, they just travel through the rectangular channel bounded by two walls. So, the instabilities should be compensated by viscosity. After they pass through the rectangular channels, they enter a region where turbulence cannot create much fluid flow difference. I used three turbulence models - laminar, k-epsilon and Transition SST and I could identify qualitative differences with respect to the convergence of species equation. The error (outflow-inflow)/inflow is greatest for laminar (about 20%), followed by k-epsilon (about 10%) and then followed by SST (About 6%). So, should I keep using SST? I do not require any analysis with respect to separation/transition.

2) I used a steady state simulation but could not reach desired accuracy with respect to the species balance. So, I am now switching to transient case. How should I decide the over the time step and the maximum number of iterations per time step?

Any help in this regard is highly appreciated. I know I have not provided many details but I will post details as you ask.